The present invention pertains generally to DC fan speed control, and more particularly, to a method and apparatus for adaptively controlling the speed of a DC fan.
Rotary fans are important components in cooling systems of electronic assemblies. Air-cooled systems typically operate by attaching heat sinks to the heat dissipating electronic components of the assemblies and generating an airflow across the heat sinks using fans.
While prior art air-cooled systems often used voltage-controlled fans in an open-loop configuration, today it is recognized that control of the fans is beneficial for both speed control and synchronization. In speed controlled systems, fan speed is usually driven by the ambient temperature inside the assemblies. By adjusting the fan speed to a speed that will meet the system cooling specifications yet less than the fan""s maximum abilities, significant audio noise reduction and power consumption reduction can be achieved. In addition, control of the fan speed to a lower speed that still meets the cooling specifications reduces wear to the fan.
It is also recognized that by synchronizing all the fans in a multiple-fan system, fan-to-fan beat frequencies can be eliminated. In addition, fan synchronization provides reduced perceived audio noise, reduced chassis vibration modes, more uniform air flow, and constant air flow over time and fan aging.
Fan speed control circuits rely on adjusting the voltage/power supplied to the fan and/or the load on the fan. In voltage controlled fans, fan speed is proportional to amount of voltage applied. Feedback from the fan""s tachometer is often used to close the loop and servo the voltage applied to the fan to more accurately control speed and sometimes phase. For DC fans, in prior art solutions, voltage adjustment was achieved using either series pass regulation or relatively high speed PWM and filtering.
Series pass voltage regulation is problematic because series pass elements are relatively inefficient. Much of the total power required to drive the fan is lost in the series pass element and therefore requires additional heat removal capability serviced usually by the fan and results in further inefficiencies to the system. Also, the additional heat often requires a heat sink to keep the control element from over-heating.
Another prior art method for controlling fan speed is the use of pulse width modulation (PWM). Using the PWM method, a voltage is fed through a switch, which applies the voltage to the fan as a train of pulses, encoding the speed of the fan in the width of the pulses. The PWM method is considerably more efficient than linear regulation because theoretically it is a lossless circuit. In a pure PWM control, the voltage input to the fan is a square wave signal with varying duty cycles. Pure PWM control is problematic because when the PWM output voltage is zero, no power is supplied to the tach sensing circuitry on the fan, and therefore no tach information is generated. One can increase the frequency of the PWM signal to get tach information with negligible error. However, this technique is problematic because conventional fans are not designed to have power applied and removed at such a high frequency. Accordingly, PWM control techniques usually include a filter which operates to smooth the PWM signal voltage to fan such that the fan does not completely lose power during normal operation. This eliminates the loss of tach information.
High speed (typically  greater than 10 KHz) PWM followed by filtering is more power efficient than series pass regulation but, as just described, requires additional cost, components (filter components including an inductor, diode, and capacitor), and board space to implement the desired function. In addition, high speed PWM can introduce significant EMC problems due to the high speed edges of the switching signals.
Accordingly, a need exists for a method and controller that controls DC fan speed without the power inefficiencies associated with the series pass regulation method or the cost associated with current PWM methods. A need also exists for a compact fan speed controller that occupies as little space as possible.
In accordance with the invention, the problems of the prior art are overcome using a novel method and apparatus for controlling the speed of a fan. In accordance with the invention, a fan controller receives a system-generated speed signal SYNC and a tachometer signal TACH from the fan. When voltage is applied to the fan, the fan rotor begins to spin and generates a tachometer signal TACH one or more times per full revolution of the rotor, each TACH signal having a fixed tach period. The controller generates a pulse-width modulated signal PWM_OUT which is used to turn the fan motor on and off. The controller adjusts the width of the PWM_OUT pulses to the fan such that the fan""s speed will either decrease or increase until the tach signal TACH matches the frequency and phase of the control signal SYNC.
In order to allow the controller to properly operate using low-frequency PWM signals, the controller synchronizes the off time PWM_OUT signal with the detection of the TACH signal and guarantees that the off time is always less than one TACH period. This ensures that the power to the fan is always turned on by the time the TACH signal arrives, and is therefore detected by the controller. Accordingly, accurate tach data is available for calculation of the next xe2x80x9coffxe2x80x9d period of the power to the fan, and pulse width modulation can be accomplished at a tach frequency of less than 200 Hz without losing any tach phase or frequency information.
In addition, the controller of the invention also generates a fan status output STATUS that represents the fan""s ability to maintain the requested speed. The status output STATUS encodes Normal, Failing, and Failed fan status. Appropriate thresholds are chosen allowing the prediction of fan failure before the actual failure occurs.